1. Field of the Invention
The present invention relates to a waveguide orthomode transducer, and more particularly, to a dual-band waveguide orthomode transducer.
2. Description of the Prior Art
Satellite communication is distinguished in wide coverage and terrestrial interference avoidance, and is widely used in military, probe, and commercial communication services, such as satellite navigation, satellite voice broadcasting, and satellite television broadcasting. A prior art satellite communication receiver consists of a dish reflector and a low noise block down-converter with feedhorn (LNBF); the LNBF is disposed on the focus of the dish reflector, receiving radio signals reflected via the dish reflector, down-converting the radio signals to middle band, and then transmitting the radio signals to a backend satellite signal processor for signal processing, enabling the playing of satellite television programs.
A single-band LNBF consists of a feedhorn, an orthomode transducer (OMT) and a low noise block down-converter (LNB), wherein the orthomode transducer is one of the key components, for separating two orthogonal polarized radio signals to be outputted from different output ports. Please refer to FIG. 1, FIG. 1 is a half longitudinal sectional view of an orthomode transducer 10 according to the prior art. The orthomode transducer 10 is a waveguide orthomode transducer composed by a rectangular waveguide 11, probes P1 and P2, and a short-circuit pin 12. The waveguide 11 is composed by four surrounding conducting walls, which is open for connecting an antenna at one end and closed at the other end. The probes P1 and P2 are formed by inner conductors of coaxial cables, passing through the conducting walls of the waveguide 11 and entering the waveguide 11. The probe P1 is parallel to the X axis, and is the output port that outputs X-polarized signals, and the probe P2 is parallel to the Y axis, and is the output port that outputs Y-polarized signals. The short-circuit pin 12 is parallel to the X axis, located at the position near the center of the waveguide 11, connecting two parallel conducting walls. The short-circuit pin 12 provides a function of polarization enabling most of the X-polarized signals be reflected and outputted from the probe P1 with little influence on the Y-polarized signals, and most of the Y-polarized signals can be successfully outputted from the probe P2.
With the growth of the needs to satellite television, the number of frequency bands covered by the direct broadcast satellite is increasing, and the prior art single-band LNBF is not sufficient anymore. The LNBF must be at least capable of receiving dual-band signals, i.e. the low frequency Ku band (12-18 GHz) and the high frequency Ka band (26.5-40 GHz) signals. Please refer to FIG. 2. FIG. 2 is a schematic diagram of a dual-band LNBF 20 according to the prior art. The LNBF 20 consists of a feedhorn 200, a low frequency orthomode transducer 202, a high frequency orthomode transducer 204 and an LNB circuit 206. The feedhorn 200 receives low frequency and high frequency radio signals. The orthomode transducer 202 separates two orthogonal polarized low frequency radio signals S1 and S2, and the orthomode transducer 204 separates two orthogonal polarized high frequency radio signal S3 and S4, making the radio signals S1-S4 be outputted from corresponding output ports to the LNB circuit 206. From the above, besides fairly separating the orthogonal polarized low frequency radio signals S1 and S2, the orthomode transducer 202 must also ensure that the high frequency radio signals S3 and S4 passing through the orthomode transducer 202 without interference as far as possible.
Nevertheless, if the dual-band LNBF 20 utilizes the orthomode transducer 10 in FIG. 1 as the low frequency orthomode transducer 202, the short-circuit pin 12 in the orthomode transducer 10 would also reflect part of high frequency X-polarized signals and output them from the probe P1, such that the high frequency X-polarized signals is unable to pass through the orthomode transducer 202 and cannot be transmitted to the orthomode transducer 204 successfully. Furthermore, since the probes P1 and P2 stretch excessively deep into the internal part of the waveguide 11, the high frequency polarized signals would also be reflected and be outputted from the probes P1 and P2; hence, not only the decay of the high frequency signals are increased, isolation between the high frequency signals and the low frequency signals is decreased. Moreover, while passing through the orthomode transducer 10, the high frequency polarized signals will be excited to higher-order modes at discontinuous probes P1 and P2. If these higher-order mode excitation signals are transmitted to the antenna, the high-frequency radiation patterns will be severely distorted. Considering the above flaws, the orthomode transducer 10 is not feasible for the dual-band LNBF 20, which possibly results in a downgrade effectiveness of the LNBF 20 when receiving the high frequency satellite signals, and hence affects the playing quality of a satellite television.